This invention is generally related to instruments of navigation and more specifically related to a speedometer.
Up to now, all the speedometers for measuring the translational speed of a moving body either are not stand-alone or do not directly measure the speed. A speedometer that is not stand-alone uses contact information outside of the moving body. For example, the speedometer in a car only works when the wheel of the car contacts with the ground, the Pitot tube of an airplane only works when the Pitot tube probes the surrounding air, the sonar of a submarine only works when sound wave reflects from some reference objects, and the GPS receiver only works when it receives the signal from the GPS satellites. A speedometer that is not directly measuring the moving speed calculates the speed based on other measurable information. For example, the accelerometer in a navigation system measures the translational acceleration and the translational speed is determined by integrating the accelerometer output with an initial speed. The present invention provides a new method and a new speedometer that is stand-alone and directly measures the translational speed of a moving body that carries the speedometer.
In accordance with one aspect of the present invention, a stand-alone speedometer for directly measuring the translational speed of a moving body comprises an electromagnetic wave source, two media with different optical properties, a beam splitter for splitting the electromagnetic beam from the source into two beams which propagate through the two media differently and a detector for measuring the travel-time difference of the two beams.
According to another aspect of the invention, a method of directly measuring the translational speed of a moving body comprises selecting two media with different optical properties, emitting an electromagnetic wave beam from a source, splitting the beam into two beams, guiding the two beams through two media differently, combining beams, measuring the travel-time difference between beams, and converting the travel time difference to the translational speed of the moving body.